Carson was rivaling Donald Trump for the lead in the Republican presidential race in mid-October, but he has since fallen to 10% in the latest CNN/ORC poll, which ties him for a distant third place with Marco Rubio, behind Trump and Ted Cruz.

His campaign has been marked of late by rumors of personnel changes and public sniping between Bennett and Watts with Armstrong Williams, Carson's business manager and sometimes public surrogate who has no official role in the campaign. Williams has criticized Bennett and Watts for not adequately preparing the candidate for public appearances, especially on foreign policy, an issue that has dominated the race in recent months.

Questions about the veracity of Carson's compelling personal story, about his rise out of poverty in Detroit to become perhaps the nation's most widely known neurosurgeon, have also hounded him as he fell in the polls.

Bob Dees, a foreign policy adviser to Carson, is now the campaign chairman. Ed Brookover, a senior strategist, will be the new campaign manager, Carson's National Finance Chairman Dean Parker said on CNN. Carson's Iowa state director Ryan Rhodes said he will remain with the campaign.

Businessman Donald Trump announced June 16 at his Trump Tower in New York City that he is seeking the Republican presidential nomination. This ends more than two decades of flirting with the idea of running for the White House.

"So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement.

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Sen. Ted Cruz of Texas has made a name for himself in the Senate, solidifying his brand as a conservative firebrand willing to take on the GOP's establishment. He announced he was seeking the Republican presidential nomination in a speech on March 23.

"These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant."

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Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid.

"I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University.

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Hillary Clinton launched her presidential bid on April 12 through a video message on social media. The former first lady, senator and secretary of state is considered the front-runner among possible Democratic candidates.

"Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey."

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Sen. Bernie Sanders, an independent from Vermont who caucuses with Democrats, announced his run in an email to supporters on April 30. He has said the United States needs a "political revolution" of working-class Americans to take back control of the government from billionaires.

"This great nation and its government belong to all of the people and not to a handful of billionaires, their super PACs and their lobbyists," Sanders said at a rally in Vermont on May 26.

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Carson said the move was needed to jumpstart his campaign.

"As we enter a new phase of the campaign cycle, it is necessary to invigorate my campaign with a strategy that more aggressively shares my vision and world-view with the American people," Carson said in a statement. "I commend Barry Bennett and Doug Watts for their efforts to help me share my vision for America. Over a year ago, hundreds of thousands of Americans encouraged me to listen to the call and seek the office of president of the greatest nation the world has ever known. I am ready to be president and believe that my unique experience and background is what is needed to heal, inspire and revive America."

Williams: "Burden" lifted

Williams said Carson has had a "burden" lifted from him with Thursday's announcement.

Carson adviser Armstrong Williams

"What Dr. Carson has to do, which he's doing now, is getting on message," Williams told CNN's Jim Sciutto on "The Lead with Jake Tapper." "He's had a major burden lift for him today. He's made the changes that he's spoke about. He's put in place a new team, and Dr. Carson's marching forward" he said. "He's on message, he's confident. I've never seen him more relaxed, more authoritarian and more in control. And that is good news as we go into 2016."

In an off-air interview with CNN, Williams said he never discussed the personnel moves with Carson.

"It was never Dr. Carson's intention for Barry to resign. The organization was changing and he was unaccepting of the change," Williams said. "And he chose to resign."

Armstrong said Dees' foreign policy experience is a "bonus, but he is also a manager who knows how to get the most out of people."

Despite the rancor, Carson's campaign has raised more than $23 million in the fourth quarter of 2015, his campaign said Wednesday, although it would not say how much cash it has on hand.

Campaign infighting

Watts, in a statement, said, "we respect the candidate and we have enjoyed helping him go from far back in the field to top tier status," later adding, "we are proud of our efforts for Dr. Carson and we wish him and his campaign the best of luck."

But Bennett and Watts have been unhappy with Williams' role with Carson, especially as Williams publicly criticized the campaign's struggles, such as the Republican Jewish Coalition speech where Carson repeatedly mispronounced the name of Palestinian group Hamas.

"His campaign has to do a better job at preparing him," Williams said in an interview with CNN earlier this month. "Don't give him speeches at the last minute. It's not only on him, it's on them."

Responding to those comments, Bennett said of Williams: "I don't think he is always helpful. You always defend your candidate."

Carson last week pushed back against reports a campaign shakeup was imminent, although he did not discount the possibility.

"The key word there is 'may.' We're always going to be looking at it. We're always going to be evaluating how people are performing" Carson told CNN's Don Lemon. "No one is ever 100% guaranteed that they're always going to be there."

The resignations were first reported Thursday by the Des Moines Register.